The major goal is to gain a understanding of transposable elements--those ubiquitous DNA elements found through nature. The experimental system in this study is the bacteria Escherishia coli K12, its bacteriophage lamda and plasmids. The transposable element is the transposon Tn5. Two aspects of this transposon's action are considered. The first is to understand the molecular mechanism of two genetic rearrangements involving Tn5 -- Tn5 transposition and precise excision. The object is to understand the molecular pathway that Tn5's DNA sequences undergo during these rearrangements and to identify the proteins that control these pathways. The second aspect is to understand the extent and possible significance of a regulatory scheme that controls Tn5's transposition. Studies on regulation and mechanism of Tn5 movement are fundamental to understanding genetic processes in all living cells. A major cause of the genetic rearrangements for any organism appears to be the transposable like elements. Understanding Tn5's mechanism of action may provide us with an understanding of a major source of spontaneous mutations that underline most genetic diseases. An implication of the work on regulation is that this major cause of genetic rearrangements is under direct genetic control; perhaps these spontaneous rearrangements are not simply mistakes of replication but rather are the result of a regulated biological system.